This invention relates generally to equalizer systems for telephone lines, and more paticularly to a two-path equalizer system, one path of which is reserved for data signals lying within a high-frequency range and the other for supervisory signals lying within a low-frequency range.
Modern telephone communication lines not only carry voice and supervisory signals which lie within a low-frequency range whose upper limit is 3 KHz, but also serve to convey data signals in a high-frequency range whose upper limit may be as high as 70 KHz. Because ordinary telephone lines have a poor amplitude response in the high frequency range, it is the present practice to interpose a single-path equalizing system at some point in the line to afford a substantially uniform signal gain over a broad frequency spectrum extending from 10 Hz to 70 KHz. By a single path system is meant one in which all signals carried by the telephone line pass through the same system regardless of their frequencies.
A telephone line equalizer is a device adapted to correct or compensate for some specified characteristic of the line which is regarded as undesirable. By the use of an equalizer, one can improve particular line characteristics and thereby enhance the quality or grade of the transmitted signal. In the case of data signals, effective use of an equalizing system may make the difference between a circuit which is acceptable and one that is effectively inoperative.
Since the losses of a telephone line are such that its amplitude-response falls off as the frequency of the signal carried thereby increases, a single-path equalization system is adapted to provide a rising gain with an increase in frequency. Proper adjustment of the equalizer is attained when the equalizer compensates for line losses over the entire band of useful frequencies.
In known forms of single-path equalizer systems which operate in the range of 10 Hz to 70 KHz, the circuit not only functions to increase gain at high frequencies to compensate for line losses but also includes an automatic-gain-control (AGC) circuit to provide a substantially constant output level despite variations in input level.
Data signals carried by a telephone line having a single-path equalizer system interposed therein contain frequency components primarily above 3 KHz, the upper limit of the voice frequency range. On the other hand, the line also carried various types of supervisory telephone tones whose frequencies lie within the voice frequency range. Thus the following supervisory signals fall within the voice frequency range: Idle Tone--2600 Hz; Dial Tone--600/120 Hz; Dial Pulse--2600 Hz, interrupted; Busy Tone--600/120 Hz; Ring Signal/Ring Back--1000 Hz; Preemption Signal--800/400 Hz.
Unlike high-frequency data signals which are yielded at the output of the equalizer system at a regulated maximum zero dBm output level, supervisory signals of the type previously exemplified are best handled at the telephone exchange when they are at their nominal signal levels. These nominal levels vary in their power level values (as expressed in decibels with reference to a power of one milliwatt) from -18 dBm to -6dBm. It is therefore desirable to provide manual gain control for the supervisory signals, so that their gain may accurately be set to make up for line losses in the low-frequency range which encompasses these signals.
Inasmuch as a single-path equalizer system makes no distinction between supervisory signals and data signals, this system precludes separate manual gain control for the supervisory signals, and therefore is incapable of supplying supervisory signals to the line at their nominal signal levels.